1. Field of the Invention
The present invention relates to a training signal detecting apparatus for detecting a repetitive pattern segment included in a reception signal.
2. Related Background Art
Hitherto, in a facsimile apparatus or the like, the modem according to V.27ter of the CCITT Recommendation and the modem according to V.29 are used.
In the V.27ter modem and the V.29 modem, in order to initialize each signal processing section of the modem on the reception side, the modem on the transmission side transmits a predetermined training signal sequence (turn-on sequence).
For instance, in the V.27ter modem and the V.29 modem, the training signal sequences are determined so as to have a construction as shown in the following tables.
TABLE 1 __________________________________________________________________________ (V.27ter training signal sequence) Segment Segment Segment Segment Segment Total of the segments 1 2 3 4 5 1 to 5 __________________________________________________________________________ Type of Non- No Continuity Pattern for Signal in Nominal time (total) line modula- trans- of adjustment which of turn-on sequence signal tion mission reversal of 2-phase continuous carrier energy of phase equalizer of "1" was by 180.degree. 0-180 scrambled 4800 bps 2400 bps Protection 185 msec 20 msec (a) 14SI (a) 58SI 8SI (a) 265 msec (a) 281 msec for 200 msec 25 msec (b) 50SI (b) 1074SI (b) 923 msec (b) 1158 msec speaker echo No 0 msec 0 msec (a) 14SI (a) 58SI 8SI (a) 50 msec (a) 66 msec protection (b) 50SI (b) 1074SI (b) 708 msec (b) 943 msec __________________________________________________________________________ SI: Symbol interval (the number of modulating times) (a): Short sequence (for convergent of the equalizer during connection of the line) (b): Long sequence (the first sync signal just after the line was set)
TABLE 2 __________________________________________________________________________ (V.29 training signal sequence) Segment 1 Segment 2 Segment 3 Segment 4 Total __________________________________________________________________________ Content of No Alternating signals Combination "1" which was -- line signal transmission of A and B signals of C and D scrambled by signal signals 1 + X.sup.-18 + X.sup.-23 The number 48 128 384 48 608 of symbols Time (msec) 20 53 160 20 253 Application Reception AGC adjustment, Initializa- Syncronization -- preparation reception carrier, tion of of scramble period after timing extraction, equalizer and descramble segment 2 synchronization, etc. __________________________________________________________________________
The reception side modem receives the training signal sequence and initializes an AGC, an automatic equalizer, and the like as a main constructing block of a receiving section.
In Tables 1 and 2, the V.27ter segment 4 and the V.29 segment 3 adjust tap coefficients at the initializing step so that the automatic equalizer which is provided in the reception side modem can realize the inverse characteristics of a communication line.
The initializing operation of the reception side modem is started from start time points of the V.27ter segment 3 and the V.29 segment 2 which are received at earlier timings than that of the segment for adjusting the equalizer.
FIG. 5A shows an absolute phase and an amplitude of a reception signal on the basis of a polar coordinate system. FIG. 5A also shows a construction of a demodulation base band signal in the segment 3 in the V.27ter 8-phase modulation 4800 bps (1600 bauds). In the diagram, Re denotes a real axis of a complex coordinate plane and Im indicates an imaginary axis. As shown by black dots in the diagram, a continuous signal comprising signals whose phases are mutually inverted by 180.degree. is received.
FIG. 5B shows frequency components of a pass band reception signal in the above segment. As shown in the diagram, the pass band reception signal at this time has line spectra at 1000 Hz and 2600 Hz as carrier frequency (1800 Hz).+-.Nyquist frequency (1600/2 Hz) of the V.27ter.
On the other hand, FIG. 5C shows a construction of a demodulation base band signal in the segment 2 in the 16-value orthogonal amplitude modulation 9600 bps (2400 bauds) of the V.29 by the same form as that of FIG. 5A. When a transmission rate is equal to 9600 bps, alternating patterns at points A and B in the diagram are received.
FIG. 5D shows frequency components of a pass band reception signal in this case. It will be understood that a carrier frequency component (1700 Hz) of the V.29 and line spectra at frequencies of 500 Hz and 2900 Hz corresponding to 1700 Hz.+-.2400/2 Hz are included in the reception signal.
A construction of a conventional training signal detecting apparatus for modem will now be described. FIG. 6 shows a signal detecting apparatus of the segment 3 in the conventional V.27ter 4800 bps training sequence. An analog reception signal which arrived at an analog input terminal 10 is sampled at a sampling frequency of 9600 Hz by an A/D converter 41 and is converted into a discrete value signal.
The discrete value signal is branched to a first pass which is squared by a multiplier 41a and a second pass which is input to band pass filters 42 and 43 having center frequencies of 1 kHz and 2.6 kHz, respectively. In the second pass, outputs of the band pass filters are squared by multipliers 42a and 43a and outputs of the multipliers 42a and 43a are added by an adder 44.
As mentioned above, an output of the multiplier 41a in the first pass indicates a power of the whole frequency components in the pass band of the reception signal, and an output of the adder 44 in the second pass indicates a power of a special frequency component which the reception signal has when it exists in the segment as a detection target.
Therefore, the outputs of both of the passes coincide when the reception signal exists in the segment as a detection target (in this case, V.27ter segment 3) while the reception signal is in the training sequence. Those outputs are different at time points other than such a case. Therefore, it is sufficient to calculate the difference between the outputs of both passes and to determine that a time point when the difference value coincides with 0 is set to a start time point of the detection target segment.
In the conventional example, in order to detect the segment start time point, a signal X which is obtained by multiplying a constant .alpha. (.alpha.&lt;1.0) to the output of the multiplier 41a is used as an output of the first pass. A difference (Y-X) between the signal X of the first pass and an output signal Y of the second pass is calculated by an adder 45. A signal Z which is obtained by adding and integrating an addition value of the adder 45 by an amount of a few samples by an adder & integrator 46 is input to a discriminating circuit (TH) 7. The reason why the addition and integration are executed by the addition & integrator 46 is to prevent an erroneous detection by caused by noise.
The discriminating circuit 47 generates a discrimination signal 48 indicative of the detection of the segment when Z&gt;0 and the non-detection of the segment when Z&lt;0. Due to this, an adjusting operation start time point of the reception side modem is determined by the discrimination signal 48.
In the above conventional example, since the segment is detected by the polarity of the difference between the power of a special frequency component and the power of the whole frequency components, the target segment can be detected in the case where an additive white noise whose power is at a uniform level for the whole frequency range is associated in the reception signal.
However, there is a drawback such that an erroneous detection of the segment easily occurs in the case where a color noise in which a peak value of the power exists near the special frequency component is associated in the reception signal or where a signal tone signal near the special frequency component is input.
On the other hand, in the above conventional example, since a segment detecting apparatus must be provided independently of and separately from the other signal processing sections of the reception side modem, there is also a problem such that a scale of the hardware of the reception side modem increases.